This invention relates to video filters, and particularly to trap filters or blocking filters of the type that are employed in a cable video system to block a premium or pay channel but to pass other channels. The invention is more specifically directed to the construction of a filter assembly which is factory tunable to a precise channel or band of channels, and which reliably stays tuned after assembly and after being installed outdoors.
Various filters have been previously employed in a television cable system environment to prevent a pay channel from reaching a subscriber who has not subscribed to that particular channel. In any of the filters previously provided for this purpose, a soldered connection to the housing was required to ensure grounding of the filter circuitry and housing. Unfortunately, soldering is a labor-intensive activity: consequently the conventional filters are somewhat expensive. In some previous filters, the filters were constructed in halves, the halves being separately tunable to achieve sufficient attenuation (preferably -70d B), and in a separate step were combined after tuning. These filters however still require internal soldering, and the soldering and assembly steps after tuning further increase production costs.
Moreover, a high failure rate for prior filter devices has kept the average price per acceptable filter unit higher than desirable.